Changes in the Electric-field (E-Field) are often unintended and unavoidable electrostatic emissions in the spectrum from sub 1 Hz to greater than 1 MHz. E-field distortions are caused by the spatial imbalance of electric charges. The electric field distortions in one instance are the result of a charge imbalance while others result from the distortion of the natural atmospheric potential gradient. Examples of E-field distortions include high voltage power lines, plasma from muzzle blasts, bullets in flight, rocket exhausts, jet afterburners. Further examples of E-field distortions include the surface friction on some materials such as plastics, helicopter rotors, and certain aerosols. Even animals and objects that are moving cause detectable distortions. For example, humans afford sufficient conductivity to measurably distort the isopotential lines of the proximate E-field as they move.
Unlike visual, infra-red, audio, and other spectra detected using prior art sensors, there has been far less research in the measurement and use of E-fields. There are also no known or readily apparent masking or countermeasure techniques or devices available for masking or concealing E-field distortions or for frustrating sensor detection of E-field distortions. As a result, the E-field sensing and processing makes practical use of spectrum that is virtually unexploited by the prior art.
TABLE ASensor type comparisonSensor typeRangeActive/PassiveBandwidthPowerSizeWeightCounter measureElectro-Optical (EO)ShortPassiveMedium-largeLowSmallLightyesInfra-Red (IR)ShortPassiveMedium-largeMediumMediumMediumyesUltra-violet (UV)Short-mediumPassiveMedium-largeLowSmallLightyesRFMedium-longPassiveSmall-largeVery lowSmallLightyesE FieldShortPassiveSmallVery lowExtremely smallExtremely lightnoRadarShort-longActiveLow-largeMedium-highMedium-largeMedium-heavyyesAcousticalShortPassiveLowVery lowVery smallLightyesSeismicShort-mediumPassiveLowLowSmall to largeMedium to heavynoMagneticShortPassiveLowVery lowSmallLightyes
As noted in Table A, several sensor types are compared according to different criteria. As shown, the sensors each have distinguishing attributes and the specific application dictates the sensor or sensor combination that accomplishes the objectives. The range may depend upon such factors as smoke, haze and noise interference, which could be appreciable in battle field conditions. The bandwidth refers to the volume of data required to relay an intelligence signal for processing. The countermeasures indication refers to present conditions, and the countermeasures typically follow advanced sensor capabilities.
There are many existing security and defense systems used for detection and identification of persons and objects, whether on the ground, underground or air-borne. Such systems are utilized whether for military applications or simple home security. The cost, complexity and installation difficulties of state of the art detection schemes make commercial implementation expensive and difficult.
Operational flexibility of sensor deployment is also somewhat limited by sensor size and power requirements. In contrast to miniaturized sensors, large sensors tend to consume more power and arc particularly disadvantageous when sensor concealment is desired. Further, active-emitter sensing devices invite counter-detection and generally consume more power than emit-on-demand and totally passive sensors.
In addition, multi-sensor systems are preferred in order to provide broader detection, provided that the sensors are small and consume little power they can cooperate with other sensors to enhance overall performance. Prior art detection schemes have incorporated varied sensors but E-field sensing has generally been ignored. And, as noted herein, many of the existing sensor types are subject to schemes to defeat the sensors.
In the home and business security area, there is considerable money spent on security systems. The sophistication and cost varies, but there is a huge volume market that is continuously seeking improvements that improve performance and lower costs. There are simple contact sensors on doors and windows that are triggered when opened. Microwave and active emission sensors continuously emit a signal and look for disturbances in the received signals. Laser and other optic systems form optical paths between sensors and are triggered when the path is blocked. While some of these sensors require fixed wire connections for power and communication, the prior art does discuss wireless embodiments. While the wired versions are more difficult to install, they provide unlimited power, even though they may be made inoperable when the connections are disconnected. The prior art systems can employ batteries, but these active systems tend to consume substantial power.
With respect to military applications, the success of existing indication and warning (I&W) sensors and related intelligence detection and collection systems are being met with increasingly sophisticated countermeasures and conscientious efforts by opposing forces on concealment and location of sensitive facilities in highly cluttered areas. Further, many critical applications in which existing multi-spectral sensors are used are also susceptible to harmful interference, jamming, and other countermeasures. For example, photographic sensors can be frustrated by deliberate concealment and weather; electromagnetic sensors by masking, jamming, and clutter; and infrared sensors by shielding and decoys.
The improvements in satellite technology providing precision visual sensing for intelligence gathering. However, many operations have placed sensitive facilities and operations out of sight and in highly cluttered areas to avoid detection. Concealment attempts require continued efforts to gather intelligence using new techniques and systems that sense unavoidable detection.
For example, typical military sensor systems use active elements such as radar to detect threats. However, active transmissions are readily detected and compromise the sensing unit by emitting detectable signals. Radar and communications signal detection devices rely on the radiation emitted intentionally by the threat source. To avoid detection, the threat platform can vary or scramble the emitted signals to avoid or confuse the detection systems. Visible and infrared emissions generated by engine exhausts and skin-friction heat are much more difficult to control than radar emissions. Although infrared does not propagate beyond the visual horizon, threat detection is adversely affected by rain, fog, clouds, and hills. Acoustic energy from engines propagates in the air, however the atmosphere substantially attenuates the acoustic signal, and thermal gradients in the air causes refraction of sound and wind cause unpredictable distortion and shadow zones.
Thus, passive detection means are preferred, as there are no emissions. As noted in the prior art, passive sensors are used to sense acoustic energy, radar frequencies, communications signals and infrared. However, each of these detection systems suffers from deficiencies.
There are numerous papers and articles that discuss some electric field sensing devices. The prior art describes simple circuits with wire antennas and a field effect transistor (FET) that perform hobbyist experiments such as detecting static electricity from combing hair and static charge created by walking across a carpet. These devices are noted as being extremely sensitive and detect all types of charges. However, the prior art is devoid of information as to refinements of detection and post measurement processing.
There have been general attempts in the prior art for sensing electric charges. One approach to passive detection is to analyze electromagnetic radiation, as described in U.S. Pat. No. 5,828,334. This invention recognizes that there is an observable phenomenon caused by the molecular kinetics in gas turbines, rocket engines and other combustion/explosive processes, where some of the gases are ionized. Any acceleration of a charge causes the electrostatic field to be distorted and electromagnetic radiation to be generated. The strength of the radiation generated by a partially ionized gas is directly proportional to the number of ions present, and correlates to the temperature, fuel rate, and the type of fuel used. The proportion of fuel molecules ionized in a simple hydrocarbon flame is exceptionally large, and the acceleration of ions causes an electromagnetic radiation with a strength that can be computed using the well-known Larmor's formula.
Ions are accelerated by several mechanisms in the engine and radiate electromagnetic waves at frequencies that are a function of the source of the acceleration. The frequencies and disturbances created by the combustion/explosion are related to mechanical characteristics. Detectable radiation from ions is derived from four causes. The first is cyclotron radiation caused by the earth's magnetic field, and the field imposes a force on the ion that accelerates it. The second source of ion acceleration and radiation is acoustic or hydrodynamic acceleration of ions that produces electromagnetic radiation, wherein this form of radiation tends to emulate the spectrum of the sounds made by the engine. The third source of acceleration of the ions is from the turbulence of the stream of gas after exiting from the engine's nozzle. As the stream from the nozzle interacts with the static gas of the atmosphere, it forms a characteristic fluid velocity field called a jet, which is a distribution of gas motion that gradually distributes the energy of the exhaust flow into the atmosphere. The fourth source of ion frequencies is mechanical acceleration of ions due to throttling in nozzles and flow alteration by turbines.
There are prior art patents for detecting missiles and aircraft, such as U.S. Pat. No. 4,703,179, which is a sensor to measure infrared or electro-optical radiation. In U.S. Pat. No. 4,765,244, there is device that measures near-infrared, far-infrared, and ultraviolet light emissions from missiles in conjunction with radar proximity information to detect and locate a missile. U.S. Pat. No. 5,430,448 senses the flicker of ultraviolet radiation in an exhaust plume and radio frequencies from nose shocks and blade-tips to detect and classify aircraft. U.S. Pat. No. 6,014,447 describes a passive vehicle identification system by analyzing the low frequency electromagnetic emanations of a vehicle engine. Another related application of sensor devices is to detect bullets and muzzle blasts. Various sensor devices such as acoustics are well known, with applicable limitations. An acoustic counter-sniper system is described in U.S. Pat. No. 6,178,141, which describes a sniper detection and localization system that uses observations of the shock wave from supersonic bullets to estimate the bullet trajectory, Mach number, and caliber. Muzzle blast observations can be used to roughly estimate a sniper location along the trajectory. The system utilizes a distributed array of acoustic sensors to detect the projectile's shock wave and the muzzle blast from a firearm. There are prior art patents that are used for determining the general direction and trajectory of projectiles. For example, U.S. Pat. No. 5,241,518 uses three spaced-apart sensors that are positioned to encounter the shock wave generated by a supersonic projectile. The sensors generate signals in response to the shock wave of the projectile that is related to the azimuth and elevation angle of a unit sighting vector from each sensor to the origin of the shock wave. A unit vector, while having direction, has no magnitude (representative of distance in this case). Another prior art system utilizes a blast wave from a fired projectile to determine the origin of the projectile, as described in U.S. Pat. No. 5,544,129, and depends on the signals generated by the transducers forming time relationships between the transducers when the blast wave serially encounters each of three required transducers. From these time relationships, at least one unit sighting vector is determined from at least one sensor to the origin of the blast wave and that unit sighting vector is considered to point in the general direction of the origin of the projectile.
However, as previously noted, the acoustic signal representative of muzzle blast can easily be corrupted after it is generated, such as by attenuation or distortion introduced by structures, e.g. buildings, topology etc, in the path of the blast as it travels toward the sensors. Similarly, in a reverberant environment, multipath arrivals of the shock wave can obscure the blast wave, which always arrives later than the shock wave. A muzzle blast waveform tends to have a lower signal to noise ratio making it difficult to precisely measure its time of arrival. Moreover, silenced weapons fire will go undetected by an acoustic system.
The Army Research Labs (ARL) have also made attempts in measuring electrostatic fields for detection purposes. The ARL discloses one detection system, wherein the ARL system is an E-field sensor system that can be incorporated into arrays with some source/signature correlation and approximating locations of the target source.
Accordingly, a need exists for l&W sensors and related systems against which existing tactics and countermeasures are less effective, and which can be operationalized and deployed at modest cost. Use of advanced low-cost, low power, low noise, high impedance amplifiers in combination with advanced digital signal processing enables reception and analysis of unintentionally created E Fields and E-field distortions. The prior art has not exploited the E-field spectrum, and tactical and technological defenses are not available to frustrate E-field sensors. The present invention extends the detection spectrum into the E-field spectrum from sub 1 Hz to greater than 1 MHz. The passive E-field sensors of the present invention can be embodied in extremely small and inexpensive devices requiring very little power, such that practical E-field sensors and E-field detection systems can be implemented.
In addition, the present system describes the system for pedestrian detection using the DC component for the walking signature and the AC component to track individual footsteps. In addition, the present system also addresses detecting certain weapon effects, small and large caliber muzzle blasts, rocket exhausts, jet afterburners, power/tension lines and motor loads. The unique measuring and processing of the present invention enable such measurements as described herein.